1. Field of Invention
This invention is related to image processing systems. In particular, this invention is directed toward a system and method that determines a spectral curve from a color sensor.
2. Description of Related Art
A typical spectrophotometer measures the reflectance of an illuminated object of interest over a plurality of light wavelengths. Typical prior spectrophotometers in this context used 16 or more channels measuring from approximately 400 nm to 700 nm, to cover the visible color spectrum or wavelength range. A typical spectrophotometer gives color information in terms of measured reflectances or transmittances of light, at the different wavelengths of light, from the test surface. This spectrophotometer desirably provides distinct electric signals corresponding to the different levels of reflected light received from the respective different illumination wavelength ranges or channels.
Another way to measure a full reflectance spectra of color samples over the entire visible range is to have monochromatic LEDs on a sensor head with the LED response curves dispersed at wavelengths where the reflectance measurement is desired. For example, in principle, to obtain 36 reflectance values each separated by 10 nm, at least 36 monochromatic LEDs would be desired on the sensor head.
However, it is not efficient to install a typical spectrophotometer, due to cost considerations, or to have 36 LEDs in every image processing system, since LEDs are not monochromatic. As an alternative, a multiple light emitting diode (LED) reflectance spectrophotometer, such as that described in copending U.S. Ser. No. 09/535,007, incorporated herein by reference in its entirety, illuminates a target with a narrow band or monochromatic light. For a low cost implementation of the color sensor based on a multiple illuminant device as the illumination source, generally, for example, 10, 12 or 16 LEDs are selected. Each LED is selected to have a narrow band response curve in the spectral space. Therefore, for example, ten LEDs would correspond to ten measurements in the reflectance curve. The LEDs, or other multiple illuminant based color sensor equivalent, e.g., lasers, are switched on one at a time as, for example, the measured media is passed through a transport of a printer. The reflected light is then detected by a photodetector and the corresponding voltage integrated and normalized with a white tile. The system and method of this invention use the integrated sensor measurements to determine a fully populated reflectance spectra with reflectance values at specific wavelengths, even though some of the light sources may not produce spectral content at the distant ends of the visible spectrum.
By using a reconstruction algorithm, based on the spectral characteristics of the illumination source and the color sensing system, the integrated multiple illuminant measurements from a non-fully illuminant populated color sensor are converted into a fully populated spectral curve. For an alternative approach to determining a spectral curve see copending U.S. application Ser. No. 09/562,072, filed herewith and incorporated herein by reference in its entirety.
The implementation of the model based reconstruction algorithm is a pre-determined look-up table that can be referenced whenever a spectral output or a L*a*b* or XYZ values are required for the sample from one of the illuminant sensors under test.
Therefore, the system and method of this invention use a spectral measurement system model to convert a number of multiple illuminant sensor voltages to reflectance values independently of the response curve of the illuminant devices.
In particular, a limited number of voltage measurements are received from, for example, a LED color sensor at the direction of a color sensor controller. The received sensor voltage measurements are then normalized in accordance with white tile measurements which are a standard practice and are well known in the color measurement industry. Next, new coefficients for a spectral reconstruction curve are determined. In particular, by using, for example, a parameterized neural network or a statistical model based on a sufficiently large training sample, the measured LED spectral emission characteristics are determined and stored in a look-up table for the LED undergoing measurement. Then, for every new measurement that is made, for example, by switching an LED on and off and measuring the reflected light, the basis vectors of the color space, the detector spectra, the integration time of the measurements and a scaling factor, which accounts for inaccuracies in the electronics of the system, are used to determine the new coefficients. Next, the basis vectors are weighted by these newly determined coefficients to produce a full spectral reflectance output curve.
For a more detailed discussion of determining basis vectors see, for example, T. Jaaskelainen xe2x80x9cVector subspace model for color representation.xe2x80x9d Vol. 7, No. 4, April 1990, J. Opt. Soc. Am. A. and J. P. S. Parkkinen xe2x80x9cCharacteristic spectra of Munsell colors.xe2x80x9d Vol. 6, No. 2, February 1989, J. Opt. Soc. Am. A., both of which are incorporated herein by reference in their entirety.
This invention provides a system and method that determine a full spectral reflectance curve.
This invention separately provides a system and method that determine a full spectral curve using measurements from a switched multiple illuminant color sensor.
This invention separately provides a system and method that determine a full spectral curve using measurements from a switched multiple LED color sensor.
This invention additionally provides a system and method in which a system model is used for spectral reconstruction.
Furthermore, it should be appreciated that the system and method disclosed in this application can be used as a foundation for determining a full reflectance curve when multiple detectors are used.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments.